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glibc/glibc-rh2213907-4.patch
DJ Delorie 8c4d8a0e5b Fix temporal threshold calculations (#2213907) 
Resolves: #221390
2023-08-08 21:25:08 -04:00

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From 180897c161a171d8ef0faee1c6c9fd6b57d8b13b Mon Sep 17 00:00:00 2001
From: Noah Goldstein <goldstein.w.n@gmail.com>
Date: Wed, 7 Jun 2023 13:18:03 -0500
Subject: [PATCH] x86: Make the divisor in setting `non_temporal_threshold` cpu
specific
Content-type: text/plain; charset=UTF-8
Different systems prefer a different divisors.
From benchmarks[1] so far the following divisors have been found:
ICX : 2
SKX : 2
BWD : 8
For Intel, we are generalizing that BWD and older prefers 8 as a
divisor, and SKL and newer prefers 2. This number can be further tuned
as benchmarks are run.
[1]: https://github.com/goldsteinn/memcpy-nt-benchmarks
Reviewed-by: DJ Delorie <dj@redhat.com>
---
sysdeps/x86/cpu-features.c | 31 ++++++++++++++++++++---------
sysdeps/x86/dl-cacheinfo.h | 32 ++++++++++++++++++------------
sysdeps/x86/dl-diagnostics-cpu.c | 11 ++++++----
sysdeps/x86/include/cpu-features.h | 3 +++
4 files changed, 51 insertions(+), 26 deletions(-)
[DJ - edited for ABI compatibility]
diff -rup a/sysdeps/x86/cpu-features.c b/sysdeps/x86/cpu-features.c
--- a/sysdeps/x86/cpu-features.c 2023-07-26 17:56:19.679300711 -0400
+++ b/sysdeps/x86/cpu-features.c 2023-07-28 15:27:00.336324265 -0400
@@ -35,6 +35,9 @@ extern void TUNABLE_CALLBACK (set_x86_sh
# endif
#endif
+unsigned long int __rtld_global_ro_cachesize_non_temporal_divisor
+ attribute_hidden;
+
#if CET_ENABLED
# include <dl-cet.h>
#endif
@@ -614,6 +617,7 @@ init_cpu_features (struct cpu_features *
unsigned int stepping = 0;
enum cpu_features_kind kind;
+ __rtld_global_ro_cachesize_non_temporal_divisor = 4;
#if !HAS_CPUID
if (__get_cpuid_max (0, 0) == 0)
{
@@ -694,13 +698,13 @@ init_cpu_features (struct cpu_features *
/* Bigcore/Default Tuning. */
default:
+ default_tuning:
/* Unknown family 0x06 processors. Assuming this is one
of Core i3/i5/i7 processors if AVX is available. */
if (!CPU_FEATURES_CPU_P (cpu_features, AVX))
break;
- /* Fall through. */
- case INTEL_BIGCORE_NEHALEM:
- case INTEL_BIGCORE_WESTMERE:
+
+ enable_modern_features:
/* Rep string instructions, unaligned load, unaligned copy,
and pminub are fast on Intel Core i3, i5 and i7. */
cpu_features->preferred[index_arch_Fast_Rep_String]
@@ -710,12 +714,23 @@ init_cpu_features (struct cpu_features *
| bit_arch_Prefer_PMINUB_for_stringop);
break;
- /*
- Default tuned Bigcore microarch.
+ case INTEL_BIGCORE_NEHALEM:
+ case INTEL_BIGCORE_WESTMERE:
+ /* Older CPUs prefer non-temporal stores at lower threshold. */
+ __rtld_global_ro_cachesize_non_temporal_divisor = 8;
+ goto enable_modern_features;
+
+ /* Older Bigcore microarch (smaller non-temporal store
+ threshold). */
case INTEL_BIGCORE_SANDYBRIDGE:
case INTEL_BIGCORE_IVYBRIDGE:
case INTEL_BIGCORE_HASWELL:
case INTEL_BIGCORE_BROADWELL:
+ __rtld_global_ro_cachesize_non_temporal_divisor = 8;
+ goto default_tuning;
+
+ /* Newer Bigcore microarch (larger non-temporal store
+ threshold). */
case INTEL_BIGCORE_SKYLAKE:
case INTEL_BIGCORE_KABYLAKE:
case INTEL_BIGCORE_COMETLAKE:
@@ -731,13 +746,14 @@ init_cpu_features (struct cpu_features *
case INTEL_BIGCORE_SAPPHIRERAPIDS:
case INTEL_BIGCORE_EMERALDRAPIDS:
case INTEL_BIGCORE_GRANITERAPIDS:
- */
+ __rtld_global_ro_cachesize_non_temporal_divisor = 2;
+ goto default_tuning;
- /*
- Default tuned Mixed (bigcore + atom SOC).
+ /* Default tuned Mixed (bigcore + atom SOC). */
case INTEL_MIXED_LAKEFIELD:
case INTEL_MIXED_ALDERLAKE:
- */
+ __rtld_global_ro_cachesize_non_temporal_divisor = 2;
+ goto default_tuning;
}
/* Disable TSX on some processors to avoid TSX on kernels that
diff -rup a/sysdeps/x86/dl-cacheinfo.h b/sysdeps/x86/dl-cacheinfo.h
--- a/sysdeps/x86/dl-cacheinfo.h 2023-07-26 17:56:18.662261475 -0400
+++ b/sysdeps/x86/dl-cacheinfo.h 2023-07-26 17:56:20.756342261 -0400
@@ -744,19 +744,25 @@ dl_init_cacheinfo (struct cpu_features *
cpu_features->level3_cache_linesize = level3_cache_linesize;
cpu_features->level4_cache_size = level4_cache_size;
- /* The default setting for the non_temporal threshold is 1/4 of size
- of the chip's cache. For most Intel and AMD processors with an
- initial release date between 2017 and 2023, a thread's typical
- share of the cache is from 18-64MB. Using the 1/4 L3 is meant to
- estimate the point where non-temporal stores begin out-competing
- REP MOVSB. As well the point where the fact that non-temporal
- stores are forced back to main memory would already occurred to the
- majority of the lines in the copy. Note, concerns about the
- entire L3 cache being evicted by the copy are mostly alleviated
- by the fact that modern HW detects streaming patterns and
- provides proper LRU hints so that the maximum thrashing
- capped at 1/associativity. */
- unsigned long int non_temporal_threshold = shared / 4;
+ unsigned long int cachesize_non_temporal_divisor
+ = __rtld_global_ro_cachesize_non_temporal_divisor;
+ if (cachesize_non_temporal_divisor <= 0)
+ cachesize_non_temporal_divisor = 4;
+
+ /* The default setting for the non_temporal threshold is [1/8, 1/2] of size
+ of the chip's cache (depending on `cachesize_non_temporal_divisor` which
+ is microarch specific. The defeault is 1/4). For most Intel and AMD
+ processors with an initial release date between 2017 and 2023, a thread's
+ typical share of the cache is from 18-64MB. Using a reasonable size
+ fraction of L3 is meant to estimate the point where non-temporal stores
+ begin out-competing REP MOVSB. As well the point where the fact that
+ non-temporal stores are forced back to main memory would already occurred
+ to the majority of the lines in the copy. Note, concerns about the entire
+ L3 cache being evicted by the copy are mostly alleviated by the fact that
+ modern HW detects streaming patterns and provides proper LRU hints so that
+ the maximum thrashing capped at 1/associativity. */
+ unsigned long int non_temporal_threshold
+ = shared / cachesize_non_temporal_divisor;
/* If no ERMS, we use the per-thread L3 chunking. Normal cacheable stores run
a higher risk of actually thrashing the cache as they don't have a HW LRU
hint. As well, their performance in highly parallel situations is
diff -rup a/sysdeps/x86/dl-diagnostics-cpu.c b/sysdeps/x86/dl-diagnostics-cpu.c
--- a/sysdeps/x86/dl-diagnostics-cpu.c 2021-08-01 21:33:43.000000000 -0400
+++ b/sysdeps/x86/dl-diagnostics-cpu.c 2023-07-26 17:56:20.761342454 -0400
@@ -117,4 +117,6 @@ _dl_diagnostics_cpu (void)
+ sizeof (cpu_features->level4_cache_size)
== sizeof (*cpu_features),
"last cpu_features field has been printed");
+ print_cpu_features_value ("cachesize_non_temporal_divisor",
+ __rtld_global_ro_cachesize_non_temporal_divisor);
}
diff -rup a/sysdeps/x86/include/cpu-features.h b/sysdeps/x86/include/cpu-features.h
--- a/sysdeps/x86/include/cpu-features.h 2021-08-01 21:33:43.000000000 -0400
+++ b/sysdeps/x86/include/cpu-features.h 2023-07-27 13:51:52.081494751 -0400
@@ -919,6 +919,10 @@ struct cpu_features
unsigned long int level4_cache_size;
};
+/* When no user non_temporal_threshold is specified. We default to
+ cachesize / cachesize_non_temporal_divisor. */
+extern unsigned long int __rtld_global_ro_cachesize_non_temporal_divisor;
+
/* Get a pointer to the CPU features structure. */
extern const struct cpu_features *_dl_x86_get_cpu_features (void)
__attribute__ ((pure));
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